A closure comprising a plastic sidewall and a metal disc is well known and a closure of this type is commonly referred to as a “combo closure”. Conventionally, the sidewall or “ring” is made from a plastic material by injection moulding. This plastic ring defines threads, which cooperate with similar threads on a container neck. A tamper evident band is defined at the free end of the “ring” and joined thereto by frangible bridges. The frangible bridges are designed to break upon opening of the closure, to indicate to a user of the closure/container package that the closure has been opened previously. Sealing compound is conventionally applied to the metal disc to form a seal between the metal disc and the mouth of the container.    Patent Citation 0001: U.S. Pat. No. 6,662,958 B (CROWN, CORK & SEAL TECHNOLOGIES CORPORATION). 2003 Dec. 16.
relates to such a closure. An advantage of this type of closure is that the tamper evident band breaks before the seal between the metal disc and the container is broken, giving a user confidence that the contents within the container has not been subject to tampering.
The 2-piece closure design allows the breaking of the container seal to be controlled. The initial twisting of the closure “ring” activates the tamper-evident feature, such as breaking a tamper-evident band, whilst the disc remains sealed to the container. Thereafter, a further feature may be provided on the closure “ring” to prise the disc from the container, thereby breaking the seal between the disc and the container to equalise any difference between the internal pressure in the container and the external environment. This 2-stage opening reduces the torque required to open the closure and allows the closure to be removed more easily.
Disadvantages of the arrangement described in    Patent Citation 0002: U.S. Pat. No. 6,662,958 B.
are that the combination of metal and plastic material makes the closure more difficult to recycle. Also, the recent increase in plastic raw material costs makes the closure relatively expensive to manufacture and the plastic “ring” prevents the use of the closure on packages, which are subjected to a retort process.
Combo closures having a metal ring confining a glass top plate (or disc) are well known in the prior art and are used for home preserving. However, these closures are also made from two materials (metal and glass), which must be recycled separately.    Patent Citation 0003: EP 1686070 A (PLATO PRODUCT CONSULTANTS). 2006 Aug. 2.
describes a “combo” closure comprising a disc and a ring shaped element. A “special feature” is described, which reduces the torque required to unscrew the closure from a container. The closure described in this document utilises a separate, annular gasket, which enhances the seal between the closure and the container to which the closure is affixed.